Pilot-wire protection of transmission lines



1965 K. R. M CLYMONT PILOT-WIRE PROTECTION OF TRANSMISSION LINES 2Sheets-Sheet 1 Filed Dec. 20, 1961 9 J ON 1532 xv. t 2 o m. i Q m 8 M K2 INVENTOR/ K. ILMCCLYMONT Awmylg s Dec. 14, 1965 K. R. M CLYMONTPILOT-WIRE PROTECTION OF TRANSMISSION LINES 2 SheetS -Sheet 2 Filed Dec.20, 1961' iNVENTOK KKMCGLYMONT Om m m2 H W m 8 W hm mm MKQQ\- V m wm o mhm wh an m g i wk 3; mw m B u wfiv: Q om m E m I I I I I I I I l l l I I1 l I l l I 1 I l l I l l l I l I I II M l X I I I l l I I l I l lATTOPLEYS.

United States Patent 3,223,891 PILOT-WIRE PROTECTION OF TRANSMISSIONLINES Kenneth R. McClymont, 29 Abinger Court, Islington, Ontario, CanadaFiled Dec. 20, 1961, Ser. No. 160,763 13 Uaims. (Cl. 317-27) Thisinvention relates to the protection of electrical transmission lines bya pilot-wire arrangement.

Pilot-wire protective arrangements presently in use are subject toincorrect operations which arise in part because of the unpredictablecharacteristics of these arrangernents. The lack of reliability of sucharrangements has specially hindered their use for the protection ofthree-terminal transmission lines where the requirements are much moresevere than for two-terminal lines.

According to the present invention there has been devised a pilot-wirearrangement which has improved performance on a two-terminaltransmission line and the behaviour of which lends itself to analyticalsolution and to the application of the arrangement to lines having threeor more terminals.

The scope of the invention is defined in the appended claims, but oneembodiment of the invention will be described in detail by reference tothe accompanying drawings in which:

FIGURE 1 is a schematic diagram of a three-phase transmission linehaving three terminals protected by a pilot-wire arrangement; and

FIGURE 2 shows in detail, for a typical terminal, the control circuitthat is indicated by one of the blocks of FIGURE 1.

The transmission line of FIGURE 1 has three terminals generallyindicated by reference numerals 1, 2 and 3, each terminal including acircuit breaker 4, 5 and 6 respectively. Three wires 7, 8 and 9 of thetransmission line are connected to terminal 1 and to wires 10, 11 and 12leading to terminal 2 and to wires 13, 14 and 15 leading to terminal 3.

In the transmission line at each terminal is a set of currenttransformers 16, 17 and 18, and the current transformers of the threeterminals are connected through control circuits 19 to pilot-Wires 20,the pilot-wires having a common junction at points 21 and thus providingan electrical path between the terminals of the transmission line.

Details of a typical control circuit 19 are shown in FIGURE 2. Let it beassumed that the circuit 19 shown in FIGURE 2 is the one associated withterminal 1 in FIGURE 1. The three-phase leads 23 and neutral lead 24 ofthe current transformers 16, 17 and 18 are connected to the primarywinding of a mixing transformer 25. The mixing transformer acts as acurrent source and has a two line output 26, 27 to which it delivers asinusoidal output current I the value of which is given by theexpression where N N and N are the numbers of turns indicated in FIGURE2 (taps on the primary winding per mitting variation of N and N I 1 andI are respectively the zero, positive and negative sequence componentsof current input to the transformer 25, and a is the vector operator 1120. It will be seen from the above expression that I is independent ofthe impedances in the circuits to which the mixing transformer isconnected.

The output lines 26, 27 of the mixing transformer are connected throughan insulating transformer 28 to the pilot-wires 20. The purpose of theinsulating transformer is to ensure that high voltages (relative toground) which may fortuitously occur on the pilotwires 20 are isolatedthere. Insofar .as the operation of the pilot-wire and control circuitsis concerned the lines 26, 27 may be considered to be continuations ofthe pilot-wires 20. The current transformers at each terminal areconnected with the same polarity as the current transformers at theother terminals so that a current normally circulates through thepilot-wires and the lines 26, 27, the current I of terminal 1 being thesum of the currents I of terminals 2 and 3.

A transformer 29 in the line 26 has its secondary winding connectedthrough a full-wave rectifier bridge 30 to a potentiometer 31, andacross the resistance of the potentiometer there appears aunidirectional restraint voltage V proportional to the line current Ipotentiometer setting may be varied to vary V over a wide range andfalse operations caused by undesired saturation of one or more of thecurrent transformers can be prevented by proper setting of thepotentiometer 31. lines 26, 27 through a full-wave rectifier bridge 33,and across the resistance 32 there appears a unidirectional operatingvoltage V proportional to the voltage across the pilot-wires.

Normally the restraint voltage V is much greater than the operatingvoltage V If, however, a fault should occur between the terminals, thecurrents in the pil0t-wires will subtract rather than add and V willbecome large compared to V By means of a line 34 the voltages V and Vare connected in opposition so that the difference voltage appearsacross leads 35; ripple in this difference voltage is substantiallyfiltered out by a series resistance 36 and shunt capacitance 37. Thetime constant of the filter 36, 37 is made small enough so as not todelay significantly the operation of the control circuit, but there canbe sufficient time delay in thefilter to prevent operation on sharptransient peaks.

The unidirectional voltage from the filter 36, 37 is applied through aresistance 38 to a transistor 39 which does not conduct unless V exceedsV If V exceeds V the transistor 39 receiving the difference betweenthese voltages begins to conduct. The transistor 39 when conductingproduces across a resistance 43 an output voltage of nearly the samemagnitude as the input voltage to the transistor, the output voltagethus being indicative of a fault between the terminals of thetransmission line. The transistor 39 acts as a power amplifier,providing a high and substantially nonvarying input impedance as seenfrom the filter 36, 37. A second transistor 44 is;

normally rendered nonconductive by a reference voltage. V It isimportant that the voltage V be held at a value which remains stableover'a long period of time, and the provision of a Zener diode 46,across which V appears, ensures such stability.

The transistor 44 is rendered'conductive when the fault indicatingvoltage across the resistance 43 exceeds V the difference being suppliedto the transistor 44 through a resistance 50. Conduction through thetransistor 44 Patented Dec. 14, 1965 The t A shunting resistance 32 isconnected across the suring that the transistor 44 remains conductive sothat the relay 54 does not vibrate. As a result, the feedback circuitensures fast operation of the line protective relay 54, of the order ofone cycle on a sixty cycle system, when the fault indicating voltageexceeds the reference voltage V The transistors and the relay 54 aresupplied from a DC. power supply indicated in FIG. 2 by its outputterminals 56, 57, 58. This power supply is independent of the currenttransformers 16, 17 and 18: that is to say, it is not supplied from thecurrent transformers and thus places no burden on them. The inputimpedance of the transistor 39 is high, and there is a high impedance inthe circuits connected across the lines 26, 27 to pro duce the restraintand operating voltages V and V Thus very little power is required fromthe current transformers to operate the relay. The transformer 29 offerslow series impedance to the circulating currents I The nature of thecomponents in the control circuit 19 is such that the performance of thepilot-wire arrangement can be predicted from a mathematical analysis.

The relay control circuit 19 is so arranged that its operation issubstantially independent of large changes in the characteristics of thetransistors. The transistor 39 is connected as a cathode follower, sothat changes in the gain of the transistor with time have negligibleeffect on its output voltage. The feedback circuit through theresistance 55 makes the gain very high, so that changes of gain of thetransistors 44 and 51 are unimportant. Of primary importance is thestability of the reference voltage V The relay 54 is caused to operatewhen the fault indicating voltage across the resistance 43 exceeds thereference voltage V and very little difference between these voltageswill operate the relay. Thus, the point at which the relay is operatedis as accurate as the accuracy of V relay operation being dependent upona comparison of the fault indicating voltage with V As an example, if Vis volts, and a fault indicating voltage of 5.1 volts will operate therelay, a loss, with time, of fifty percent of the gain through thetransistors 44 and 51 would necessitate an increase in the faultindicating voltage to only 5.2 volts to operate the relay.

The reference voltage V provides a sharp point of operation for therelay 54. Whereas the-voltage at which a given relay will pick up canvary substantially, the control circuit forthe relay operates preciselyand then provides ample power, not drawn from the current transformers,to operate the relay coil. It will be noted that the voltage V is notprovided through the current transformers; rather the means forsupplying V are the D.C. power supply and the Zener diode 46 which areindependent of the electrical output of the current transformers.

Typical components for the relay control circuit are as follows:

Rectifier 30 1N207l. Rectifier 33 t 1N2071. Capacitance 37 /2microfared. Resistance 31 100 kilo-ohms. Resistance 32 15 kilo-ohms.Resistance 36 22 kilo-ohms. Resistance 38 27 kilo-ohms. Resistance 438.2 kilo-ohms. Resistance 50 30 kilo-ohms, Resistance 52 1.5 kilo-ohms.Resistance 53 2.2 kilo-ohms. Resistance 55 320 kilo-ohms, Terminal 56 0volt. Terminal 57 45 volts. Terminal 58 18 volts. V 5.0 volts.Transistor 39 2N365. Transistor 44 2N365.

Zener Diode 46 1N750A. Transistor 51 t 2N251.

Transformer 25 may be a current transformer for which N :5 amps, N021amp. and N :0.01 amp. Insulating transformer 28 may have a 1:1 ratio,and transformer 29 a 1:27 ratio. The relay 54 may be a 350 ohm relayrated to operate at 24 volts.

Vacuum tubes could of course be used instead of transistors, and in theappended claims the term valve will be understood to include a vacuumtube or a transistor. Protective and compensating devices ofconventional design will normally be provided. For example, tocompensate for the capacitance of the pilot-wires adjustable reactorscan be connected across the junction points 21 and across the lines 26,27 at the insulating transformers 28, and it may also be desirable toprovide potentiometers across the lines 26, 27 at the transformers 28 tobalance the open circuit transfer impedances of the pilot-wires. Tolimit the voltages applied between the pilot-wires 20, voltageregulating gaseous discharge tubes can also be provided across the lines26, 27 at the insulating transformers 28.

What I claim as my invention is:

1. A pilot-wire arrangement for protecting a three phase electrictransmission line having a plurality of terminals, comprising a linecircuit breaker at each terminal, a relay for each terminal for openingthe circuit breaker of the terminal, current transformers in the line ateach terminal, pilot-wires providing an electrical path between theterminals, and a relay control circuit for each terminal withconnections between the pilot-wires and the current transformers of theterminal of such polarity that a current normally circulates in thepilot-wires, the con nections of each control circuit including a mixingtransformer having an input from the current transformers of theterminal and a two line output for delivering a current which is afunction of the positive, negative and zero sequence components of thecurrents from the current transformers and is substantially independentof the impedance of the pilot-wires, and an insulating transformerconnected between the two line output and the pilotwires, meansconnected across the two line output for providing an operating voltagedependent on the voltage across the pilot-wires, a transformer in oneline of the two line output and means connected across the lastmentioned transformer for providing a restraint voltage dependent on thecurrent in the pilot-wires, means responsive to a predetermineddifference between the operating voltage and the restraint voltage toprovide a voltage indicative of a fault between the terminals of theline, a power supply independent of the current transformers, meansproviding a stable reference voltage, and an elec tronic amplifier foroperating the relay of the terminal from the independent power supply inresponse to a predetermined difference between said indicative andreference voltages, thus opening the circuit breaker of the terminal.

2. A pilot-wire arrangement for protecting a threephase electrictransmission line having a plurality of terminals, comprising a linecircuit breaker at each terminal, a relay for each terminal for openingthe circuit breaker of the terminal, current transformers in the line ateach terminal, pilot-wires providing an electrical path between theterminals, and a relay control circuit for each terminal withconnections between the pilot-wires and the current transformers of theterminal of such polarity that a current normally circulates in thepilot-wires, the connections of each control circuit including a mixingtransformer having an input from the current transformers of? theterminal and a two line output for delivering a current which is afunction of the positive, negative and zero sequence components of thecurrents from the current transformers and is substantially independentof the im-- pedance of the pilot-wires, and an insulatingtransformerconnected between the two line output and the pilotwires, ashunting resistance connected through a rectifier across the two lineoutput for providing a unidirectional operating voltage dependent on thevoltage across the pilot-wires, a transformer in one line of the twoline output and a resistance connected through a rectifier across thelast mentioned transformer for providing a unidirectional restraintvoltage dependent on the current in the pilot-Wires, a D.C. power supplyindependent of the current transformers and of the pilot-wires, a D.C.electronic power amplifier supplied by the independent power supply, afilter delivering the difference between the operating and restraintvoltages to the amplifier, the amplifier being responsive to apredetermined difference between the operating voltage and the restraintvoltage to provide a D.C. output voltage indicative of a fault betweenthe terminals of the line, means providing a stable D.C. referencevoltage, and a D.C. electronic amplifier supplied by the independentpower supply and operative in response to a predetermined differencebetween said indicative and reference voltages to operate the relay ofthe terminal and thus open the circuit breaker of the terminal.

3. A pilot-wire arrangement for protecting an electric transmission linebetween terminals of the line, comprising (1) a line protective relayfor each terminal;

(2) current transformers in the line at each terminal;

(3) pilot-wires providing an electrical path between the terminals; and

(4) a relay control circuit for each terminal, the relay control circuitcomprising (a) means so connecting the current transformers with thepilot-wires that the pilot-wires carry a circulating current dependentupon the currents in the line and substantially independent of theimpedance of the pilot-wires,

(b) means for deriving from the circulating current a signal indicativeof a fault between the terminals of the line,

(c) means for providing a reference signal,

(d) means for comparing said signals, and

(e) means responsive to a predetermined comparison between said signalsfor operating the relay for that terminal to protect the line.

4. A pilot-wire arrangement as claimed in claim 3, wherein the meansconnecting the current transformers with the pilot-wires comprise amixing transformer having its input connected to the currenttransformers and having a two line output supplying the pilot-wires.

5. A pilot-wire arrangement as claimed in claim 3, wherein the means forproviding the reference signal comprise means independent of theelectrical output of the current transformers for producing a stablereference signal.

6. A pilot-wire arrangement as claimed in claim 5, wherein the means forderiving from the circulating curcurent a signal indicative of a faultcomprise (i) means for providing a voltage dependent upon thecirculating current,

(ii) means for providing a voltage dependent upon the voltage across thepilot-wires, and (iii) means for comparing said voltages to derive saidsignal indicative of a fault.

7. A pilot-wire arrangement as claimed in claim 6, wherein the means forproviding a voltage dependent upon the circulating current comprise atransformer through the primary winding of which the circulating currentflows and across the secondary winding of which the last mentionedvoltage is developed.

8. A pilot-wire arrangement as claimed in claim 6, wherein the voltagecomparing means (iii) comprise an electronic valve, saidvoltage-providing-means (i) and (ii) being connected in opposition atthe input of said valve, said valve being responsive to an excess of oneof said voltages over the other to provide said signal indicative of afault.

9. A pilot-Wire arrangement as claimed in claim 8, wherein theelectronic valve comprises a ower amplifier.

10. A pilot-wire arrangement as claimed in claim 5, wherein the meansresponsive to said predetermined comparison comprise an electronic valvehaving its input from said signal comparing means and normally biasedoff by the stable reference signal but rendered conductive by apredetermined difference between the signals, the relay being in anoutput circuit of the valve.

11. A pilot-wire arrangement as claimed in claim 10, including afeedback circuit from the output circuit to the input of the valve formaintaining the valve conductive.

12. A pilot-wire arrangement as claimed in claim 5, wherein the relaycontrol circuit includes a power supply independent of the electricaloutput of the current transformers and wherein the relay operating meansenergizes the relay from the independent power supply.

13. A pilot-wire arrangement as claimed in claim 12, wherein the powersupply is a D.C. power supply and said signals are D.C. signals.

References Cited by the Examiner UNITED STATES PATENTS 2,242,950 5/ 1941Harder 31727.2 2,406,411 8/ 1946 Sonnemann 3 l727.2 3,037,151 5/1962Cimerman et al. 317-27 SAMUEL BERNSTEIN, Primary Examiner.

1. A PILOT-WIRE ARRANGEMENT FOR PROTECTING A THREEPHASE ELECTRICTRANSMISSION LINE HAVING A PLURALITY OF TERMINALS, COMPRISING A LINECIRCUIT BREAKER AT EACH TERMINAL, A RELAY FOR EACH TERMINAL FOR OPENINGTHE CIRCUIT BREAKER OF THE TERMINAL, CURRENT TRANSFORMERS IN THE LINE ATEACH TERMINAL, PILOT-WIRES PROVIDING AN ELECTRICAL PATH BETWEEN THETERMINALS, AND A RELAY CONTROL CIRCUIT FOR EACH TERMINAL WITHCONNECTIONS BETWEEN THE PILOT-WIRES AND THE CURRENT TRANSFORMERS OF THETERMINAL OF SUCH POLARITY THAT A CURRENT NORMALLY CIRCULATES IN THEPILOT-WIRES, THE CONNECTIONS OF EACH CONTROL CIRCUIT INCLUDING A MIXINGTRANSFORMERS HAVING AN INPUT FROM THE CURRENT TRANSFORMERS OF THETERMINAL AND A TWO LINE OUTPUT FOR DELIVERING A CURRENT WHICH IS AFUNCTION OF THE POSITIVE, NEGATIVE AND ZERO SEQUENCE COMPONENTS OF THECURRENTS FROM THE CURRENT TRANSFORMERS AND IS SUBSTANTIALLY INDEPENDENTOF THE IMPEDANCE OF THE PILOT-WIRES, AND AN INSULATING TRANSFORMERCONNECTED BETWEEN THE TWO LINE OUTPUT AND THE PILOTWIRES, MEANSCONNECTED ACROSS THE TWO LINE OUTPUT FOR